Single-light Emitters in the Confinement of Polymers

This thesis presents developments in the field of nanoscale probing of polymers. New experimental methods based on single molecule fluorescence detection were developed and applied to polymer studies. Localization of, and communication with individual fluorescent molecules embedded in a glassy polymer or immersed in a polymer melt have been realized by using various, optical techniques e.g. Scanning Confocal Microscopy and Wide-Field Microscopy. Location of single molecules as well as their orientation, emission spectra or fluorescence lifetime have been followed in time. This allowed us to perform dynamical studies in the time domain. Since full distributions of single molecule behavior (e.g. rotational diffusion constants, fluorescence lifetimes) were obtained, systems with heterogeneous dynamics could be clearly identified and sub-ensembles, which could be characterized with different dynamic properties, were separated and investigated. Besides polymer probing studies, it was also shown how relatively simple structures can be prepared and used to engineer the emission properties of single molecules. In particular, the fluorescence emission of single molecules in thin polymer films and in electrospun polymer fibers was shown to depend on the size of the structures. The following paragraphs give a more detailed summary of the results. Chapter 2 and Chapter 3 were introductory chapters. Introduction to topics in polymer physics is presented in Chapter 2. A special accent was put on the reviewing of the current status in the investigations of the dynamic properties of polymers at interfaces and in different degree of confinement. In Chapter 3 single molecule fluorescence methods were introduced and their application in polymer studies was reviewed. The need for the development of new experimental techniques to investigate polymers on the nanoscale was underlined. Techniques based on optical single molecule detection were shown to be the most promising candidates to resolve the controversies present in the polymer field concerning segmental scale dynamics or heterogeneous dynamics near the glass transition. Wide-field microscopy was employed to investigate polymer dynamics across the glass transition and the results of these investigations were presented in Chapter 4. Far above the glass transition temperature translational diffusion of single molecules was observed and translational diffusion constant were extracted. At temperatures close to the glass transition, using light polarization detection scheme, the orientational diffusion of many different single probes was followed simultaneously. The molecules were shown to be immobile, to hop between different sites within the matrix, or to diffuse freely on different time-scales. Such observations gave a significant insight into the heterogeneous dynamics present in a polymer host above and near the glass transition temperature. It was also found that the nonexponential character of the single molecule diffusion processes near the glass transition was a result of the changes in the probe behavior in time. Below the glass transition temperature the changes of the photophysical properties of single emitters in response to the changes in the environment were monitored and gave a valuable insight into the structural heterogeneity present in a glassy polymer matrix. In Chapter 5 density fluctuations in a glassy polymer matrix were detected through the monitoring of single molecule fluorescence lifetime fluctuations. Using this newly developed method, direct information on the local, nanoscale host dynamics was obtained. From the experimental data, using the Simha - Somcynsky thermodynamic equation of state, we were able to obtain the number of polymer segments (NS) taking part in the elementary rearrangement processes around the probe. For two different polymers, polystyrene and poly(isobutyl methacrylate) NS was a function of temperature and it decreased with increasing temperature. Interestingly, it was found that NS has similar temperature dependence for different polymers when normalized with respect to the glass transition temperature of the polymer used. A unique combination of small probed volumes and nonensemble measurements allowed us to directly visualize the spatially heterogeneous dynamics present within a polymer matrix below the glass transition temperature. Such a result is otherwise difficult, if not impossible, to achieve using other experimental techniques. In planar, stratified media (e.g. glass/polymer/air) consisting of optically different materials the fluorescence emission of single molecules can be strongly modified due to the near-by presence of electromagnetic boundaries. The radiative decay rate in these cases should be dependent on the size of the layers, the distance of the molecule to the interfaces and to the orientation of the molecule emission dipole moment with respect to the interfaces. Before starting to investigate segmental scale dynamics in polymer structures with dimensions comparable to the wavelength of light it is important to perform a check whether the optical characteristics of the chromophoric probes are not influenced by the electromagnetic boundary conditions. In Chapter 6 we showed that the fluorescence lifetime of single molecules embedded in thin polymer films changes depending on the thickness of the films and increases when the film thickness decreases. Calculations of fluorescence lifetime of light emitters embedded within layered structures have shown that the presence of the interfaces is the main actor in the modifications of the fluorescence lifetime. Although important to realize, such "global" effects were found to not significantly affect the method presented in Chapter 5 if the chromophores are not reorienting within the polymer matrix or translating with respect to the interfaces. As shown in Chapter 4 and Chapter 5 this was not the case for molecules in a polymer like polystyrene at room temperature In Chapter 7 we used the single molecule lifetime fluctuation technique to study the influence of the film thickness on local polymer dynamics on the nanometer length scale at temperatures far below the glass transition temperature. We found modified segmental scale dynamics when the polymer was confined into films with thickness below 60-70 nm corresponding to 6 times the radius of gyration of the polymer used. This behavior was attributed to the effect of the polymer free surface with enhanced dynamics. The effects of the interfaces propagated deep into the polymer sample over distances larger than the radius of gyration. Investigations of hybrid polymer/light emitter nanofibers made of glassy or semicrystalline materials (PMMA, PEO) prepared by electrospinning was presented in Chapter 8. Organic molecules, as well as semiconductor nanocrystals (quantum dots), have been incorporated into the structures at different concentrations down to the single molecule level. The electrospinning technique has been proven to be a good and reliable method to prepare quasi one-dimensional luminescent structures on length scales ranging from tens of nanometers to several microns. The fluorescence lifetime distributions of single molecules embedded in PMMA fibers were shown to become broader for fibers with diameters below 500 nm. This was attributed to the effect of the interfaces on the excited state lifetime of the chromophores. Bead-on-a-string morphology of electrospun polymer fibers was obtained. The structure of the beads was found to be dependent on the polymer used to spin. In particular, when a semicrystalline polymer was used for electrospinning, the fluorescent molecules were excluded from the bead interior probably due to the growing polymer crystals. Additionally, there was evidence that the molecules adopt a specific orientation at the bead edges. The beads displayed also interesting photonic properties, which could be used in future applications based on advanced polymer structures. In Chapter 9 a brief outlook into the possible future research directions was presented. The necessity of an in-depth follow-up of the methods presented throughout the thesis was underlined. We are confident that the research presented in this thesis will prove to be powerful in the investigations of various polymers systems or complex polymer structure e.g. block copolymers or polymer networks.